This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The survival of all organisms depends critically on the ability to faithfully replicate DNA. However, cellular DNA is continually subjected to damaging agents such as UV and ionizing radiation externally, and oxidation and hydrolysis internally. How cells bypass these lesions during DNA replication has been a key question in the areas of DNA replication, mutagenesis, and carcinogenesis. The answer to this longstanding puzzle came only come relatively recently with the discovery of DNA polymerase [unreadable] (Pol[unreadable]). Pol[unreadable] is unique amongst eukaryotic DNA polymerases in its proficient ability to replicate through a UV-induced cis-syn cyclobutane thymine-thymine (T-T) dimer) by inserting two adenines opposite the dimer. Mutations in human Pol[unreadable] are responsible for an inherited cancer-prone disorder, the variant form of xeroderma pigmentosum (XP-V). Pol[unreadable] is, thus, the first DNA polymerase demonstrated to act as a tumor suppressor in humans. We have determined the structure of apo-Pol[unreadable], but a structure of Pol[unreadable] with bound DNA has remained elusive. This has greatly hindered an understanding of how Pol[unreadable] accommodates DNA lesions and the molecular basis of XP-V mutations in humans. After an exhaustive effort, we have finally succeeded in cocrystallizing Pol[unreadable] with a DNA template-primer and an incoming nucleotide. However, the cocrystals present a challenge: attempts to measure X-ray data have been greatly hindered by the very long c axis (634.7[unreadable]). The highly focused X-rays at beamline 24-ID will enable us to measure X-ray to at least ~4[unreadable], and possibly to an even better resolution from these large unit cells cocrystals.